A flash memory cell consists of an MOS transistor having a floating gate. The threshold voltage of the MOS transistor may be changed repeatedly by applying an electric field on its gate. This operation process is referred to as programming. Corresponding to the charges existing in the floating gate, the memory cell may have two threshold voltages, i.e., two states. When electrons in the floating gate accumulate, the threshold voltage of the memory cell may be increased, because the pre-charge level of the bit line and the read signal voltage applied to the control gate remain constant, and the memory cell is not turned on. The threshold voltage of the memory cell may be lowered by removing electrons from the floating gate. In this case, the signal voltage and the bit line used are grounded and discharged, and the transistor of the memory cell is turned on.
Analysis and Design on CMOS Digital integrated Circuits (3rd Edition), Publishing House of Electronics Industry, January 2005, discloses a method for programming flash memory cells, which comprises: increasing the threshold voltage by storing electrons to the floating gate of the MOS transistor of a flash memory cell via channel hot electron injection; and making the floating gate of the MOS transistor release electrons with tunneling mechanism, thus data programming can be performed on the flash memory cell. According to the principle of such a programming method, an electrical performance index of the flash memory cell, i.e. the electron accumulation ability of the floating gate, may be inspected by measuring the threshold voltage of the flash memory cell. That is, after a certain time period of electron-storage programming, whether the floating gate of the flash memory cell can accumulate sufficient electrons so as to make the threshold voltage reach a specified reference value. Therefore, in the existing inspection method, an inclement external environment is simulated to inspect a flash memory cell according to the practical application requirements. The existing inspection method is divided into four steps: electron-storage programming, pre-inspection, application simulation and final-inspection. As shown in FIG. 3, the electron-storage programming is to perform an electron-storage programming on a flash memory cell, so that the threshold voltage of the flash memory cell may be increased. The pre-inspection is to measure the threshold voltage of the flash memory cell, and if the threshold voltage obtained by measuring the flash memory cell reaches a specified reference value, it indicates that the electron accumulation ability of the floating gate of the flash memory cell is preliminarily acceptable, otherwise, it indicates that the electron accumulation ability of the floating gate of the flash memory cell is not acceptable, that is, the electrical performance is not acceptable. The application simulation is to simulate a practical application by establishing an inclement external environment using the batch of preliminarily acceptable flash memory cells screened out by the above method, so and so only can flash memory cells suitable for practical application be screened out. To establish a simulated inclement external environment, the flash memory cells are usually baked. The final-inspection is to measure the threshold voltage of the batch of preliminarily acceptable flash memory cells again and find out whether it can still be maintained at or above the specified reference value; if the threshold voltage of the flash memory cell drops below the specified reference value, then it is considered that the flash memory cell cannot endure practical application, and the electrical performance of such flash memory cells will be regarded as unacceptable, otherwise, the flash memory cell will be regarded as acceptable for practical application, and the electrical performance of such flash memory cells will be regarded as acceptable.
However, during baking, the threshold voltage of the flash memory cell may drop in some sort. As shown in FIG. 1, for some flash memory cells having a floating gate with strong electron accumulation ability, sufficient electrons may be accumulated during electron-storage programming, so that the threshold voltage is very high and it can still be maintained highly above the specified reference value even after it drops due to the baking during application simulation. However, for some flash memory cells of which the electrical performance is in a critical state, the threshold voltage is just at about the specified reference value after electron-storage programming; and the threshold voltage of these flash memory cells may drop below the specified reference value after baking. Thus, the electrical performance of these flash memory cells may inevitably be regarded as unacceptable, for their threshold voltage cannot reach the specified reference value. However, in fact, the electron accumulation ability of these flash memory cells is usually acceptable. Therefore, the prior art technology has the following defect: a large number of flash memory cells of which the electrical performance is in a critical state cannot pass the second threshold voltage inspection, and are considered as products with unacceptable electrical performance, thus the yield may be decreased.